1. Field of the Invention
The present invention relates to a material testing machine which measures the hardness, tensile strength, yield stress, Young's modulus, creep strength, or the like of a material (which hereinafter referred to simply as the hardness of the material, in the interests of brevity) by pressing an indenter into the surface of the material (hereinafter referred to as a specimen) to make therein an indentation to a predetermined depth. More particularly, the invention pertains to a material testing machine which measures the hardness of a specimen through utilization of either one or both of the amount of penetration of the indenter into the specimen, which corresponds to a first predetermined value of the pressure on the specimen when the indenter is pressed thereinto to the predetermined depth, and the amount of penetration which corresponds to the above-mentioned predetermined first value or a second value of the pressure on the specimen when the indenter is pulled up form the specimen.
2. Description of the Prior Art
As a material testing machine of this type there has been proposed a machine which is equipped with a specimen table for holding a specimen, an indenter for impression into the specimen to make therein an indentation, an indenter pressing member for pressing the indenter into the specimen, indenter pressing force detecting means for detecting the pressure on the specimen form the indenter, and amount of penetration detecting means for detecting the amount of penetration of the indenter into the specimen. In this material testing machine, the amount of penetration detecting means has a displacement detector provided with a stator and a movable element. The stator is fixed to either one of the indenter and the specimen table and the movable element is contacted at its free end with the other one of them.
With such a material testing machine, the pressure on the specimen can be measured using the detected output from the indenter pressing force detecting means and the amount of penetration of the indenter into the specimen can be measured using the detected output from the amount of penetration detecting means not only when the indenter is pressed into the specimen to make therein an indentation but also when the indenter is pulled up away from the specimen. Accordingly, the hardness of the specimen can be measured.
In the conventional material testing machine, however, the indenter pressing force detecting means is complex in structure, imposing severe limitations on the reduction of the whole size on the machine and its manufacturing cost.
Furthermore, according to the prior art machine, since the stator of the displacement detector of the amount of penetration detecting means is fixed to the specimen table or the free end of its movable element is held in contact with the table, the detected output from the displacement detector is affected by the condition of the surface of the specimen table and the condition in which the specimen is held on the table. Moreover, the detected output from the displacement detector of the amount of penetration detecting means contains an error which results from the lowering of the specimen surface relative to the table surface which is caused by the pressure on the specimen from the indenter. Therefore, the amount of penetration of the indenter into the specimen cannot be measured with high accuracy.
Accordingly, the conventional material testing machine cannot accurately measure the hardness of the specimen.